Hearing aid measurements are performed by using a closed fitting (occluded fitting). Reference can be made to IEC 60118 series of standards or the ANSI S3.22 standard. Closed fitting means that the acoustic output of the hearing aid is sealed to the measurement coupler or ear simulator. The couplers used for hearing aid measurements are described for example in the IEC 60318 series of standards.
During the last couple of years, so called open fitted hearing aids have become very popular. Examples are: thin tube instruments with dome; or Receiver In the Canal Instruments with dome.
It is desirable to measure hearing aid performance in open fitting conditions, in which INSITU conditions are simulated as much as possible. Closed fit measurement provides a response which deviates very much from the INSITU conditions.
It has been proposed to measure in closed fitting conditions and then apply post-measurement correction curves.
The problems with non-occluded coupler measurements are for example:                a) summation of direct sound and processed sound results in an output frequency response with sharp notches (comb filter effect);        b) the sound leaking from the non-occluded coupler or ear simulator may lead to acoustic feedback which also distorts the output signal (frequency response); and        c) the summation in a) depends on the orientation and placement of the coupler and hearing aid relative to the sound source and processing delay of the circuit.        
The problems have led to the conclusion that open fit measurements are not repeatable and thus not practical.